Two police officers in Georgia have been fired after videos showed them separately punching a 21-year old motorist in the face and then kicking him in the head as he lay on the ground in handcuffs during a traffic stop, officials said.

The Gwinnett County Police Department said in statements on Thursday that a “very disturbing” cellphone video taken by a witness showed Officer Robert McDonald, 25, using “unnecessary and excessive” force when he kicked the handcuffed man in the head on Wednesday.

A second cellphone video was later discovered by the police online, showing Sgt. Michael F. Bongiovanni, 41, hitting the victim, who was getting out of a car with his hands up, in his face. The sergeant then lied about the encounter, the department said.

The department said it had started a criminal investigation into the actions of both officers. A representative of the district attorney’s office was not immediately available on Friday to comment on whether charges would be filed.

“The revelations uncovered in this entire investigation are shocking,” the police department said. “We are fortunate that this second video was found and we were able to move swiftly to terminate a supervisor who lied and stepped outside of his training and state law.”

Sergeant Bongiovanni has been with the department since 1998, and Officer McDonald was hired in 2013, the department said. The two could not be reached by telephone on Friday.

The episode unfolded at 4:30 p.m. on Wednesday near Lawrenceville, a suburb of Atlanta and the county seat of Gwinnett County, which has a population of about 850,000. After the videos were shared online, it quickly became another example of cellphone videos recorded by bystanders prompting investigations of police behavior that fuel mistrust between mostly white law enforcement officers and black people.

In another example this week, a Sacramento police officer, who is white, threw a black man to the ground after stopping him on suspicion of jaywalking, then pinned him there before striking him in the face at least a dozen times, a video showed. The department said that a special unit was investigating and that the Sacramento County district attorney’s office would decide whether to file charges. That officer was put on leave.

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The Gwinnett police department said in its statements that it was aware of the impact such encounters have on its community. “We acknowledge that the actions of these two officers have implications that will be felt for some time,” it said.

According to a report on the traffic stop that was emailed on Friday by a police spokeswoman, Cpl. Michele Pihera, Sergeant Bongiovanni, who is white, tried to pull over the red Acura Integra driven by the man, Demetrius Bryan Hollins, who is black. The report said that the vehicle did not have a license plate, that a brake light was broken and that Mr. Hollins had changed lanes several times without signaling.

Mr. Hollins slowed down but did not pull over until the car stalled, the report said.

Sergeant Bongiovanni’s report said that the car had smelled of marijuana and that Mr. Hollins had begun to act strangely, saying, “I need to call my mom.” Sergeant Bongiovanni asked Mr. Hollins to get out of the car, and he ignored the request, the report said.

Sergeant Bongiovanni, who said he had called for backup after recognizing Mr. Hollins from a 2016 arrest on a charge of having a loaded gun, eventually used a Taser on Mr. Hollins, pinned him on the ground and handcuffed him, the report said.

One of the cellphone videos captures about 45 seconds of that interaction, according to footage published by WSB-TV and other local media. After Mr. Hollins is lying on the pavement, the video shows Officer McDonald running up and, without pausing, stomping or kicking Mr. Hollins in the head.

Mr. Hollins could not be reached by telephone on Friday.

Another video shot by a witness from a different location shows Sergeant Bongiovanni punching Mr. Hollins in the head as he was getting out of the car with his hands up.

Sergeant Bongiovanni’s latest performance review, provided by the police department, showed that by most measures, he often exceeded expectations as a law enforcement officer.

Mr. Hollins was charged with possession of less than an ounce of marijuana, obstruction of a police officer and traffic violations, the incident report said. Corporal Pihera said in an email that Mr. Hollins had been released from jail on bond and that the removal of the officers from their jobs would be permanent.

Mr. Hollins told NBC News on Friday that he had been trying to reach a camera app while in the car because of his previous “encounter” with Sergeant Bongiovanni, who “charged me with the same charges as he charged me with yesterday.” He said the sergeant had started “shoving me in my car and telling me that I was never going to have a video, that I was never going to make the phone call to my mom.”

“When I had my hands up, that’s when he punched me in the face,” he told NBC.